Guido de Marco, who has died aged 79, rose from humble beginnings to become a top criminal legal counsel, a highly respected politician and statesman, the sixth president of Malta and, in 1990, the president of the general assembly of the United Nations.

Persuasion and perseverance characterised de Marco's career, which spanned more than five decades. However, he also recognised the dignity and intrinsic value of each and every person he met. I first met him when I was nine, during a family visit. He spoke about Julius Caesar, whom de Marco admired because he had changed history. Indeed, de Marco often referred to history as a guide to change.

De Marco was born to a working-class family in Valletta, the capital of Malta, the Mediterranean island group which was then a crown colony. He graduated as a lawyer in 1955 and quickly became the leading criminal counsel in Malta, then passing through very trying times. The prime minister Dom Mintoff's integration plan had failed, a Break With Britain resolution was adopted and social unrest led to the suspension of the constitution in 1958. De Marco was against integration and looked forward to Malta's self-determination. After an unsuccessful stint in the 1962 general elections, he campaigned for Malta's independence, winning favour from the Nationalist party (NP) leader and prime minister George Borg-Olivier.

The NP won the 1966 general elections, held just 18 months after Malta became independent, and de Marco was elected for the first time. In the 1970s and 80s, the NP was in opposition. During these tough times, he played a crucial role. As deputy leader of the party, he helped to transform it from a conservative to a more diverse, open organisation.

In 1987, together with Mintoff, de Marco helped resolve a serious political impasse resulting from the 1981 election result, which saw the NP gain the absolute majority of the votes (51%) but a minority of the seats. The subsequent constitutional changes helped the NP to power; de Marco became deputy prime minister and minister for justice and home affairs.

In May 1990, he became minister for foreign affairs and that September was also elected president of the UN general assembly. Just a few weeks before, Saddam Hussein had invaded Kuwait. "We will fight for your cause in peace," de Marco told the Emir of Kuwait, who had just addressed the general assembly. Twenty years on, many Kuwaitis still remember and cherish de Marco's stand for their country.

That summer, as foreign affairs minister, he also submitted Malta's bid for membership of the European Union. It proved to be a long, arduous road, riddled with obstacles, both from the opposition Labour party, as well as from some EU members. In his autobiography The Politics of Persuasion (2007), de Marco expressed his regret and frustration at the way Malta was treated. He recalled how the former French prime minister Alain Juppé changed his views about Malta's EU bid when de Marco told him that he was due to meet Yasser Arafat and Shimon Peres in a bid to bring peace to the Holy Land. It took de Marco more time and patience to win over the UK foreign minister Douglas Hurd.

The NP's quest for EU membership took another blow in 1996, when it lost the general elections and a revamped Labour party froze Malta's application. Labour's tenure was short-lived and it lost power after a vote of confidence. The NP won elections in 1998 and, just four days later, through de Marco, reactivated its application. Again, in the light of the previous administration's attitude, he faced a difficult task convincing many EU stalwarts (and the local population) about Malta's determination to join. When he became president of Malta, he tried to unite the polarised, partisan island and in 2004, after a successful referendum, Malta became an EU member.

As the president of the UN general assembly, apart from his open support for Kuwait, de Marco will also be remembered for his stand for Palestine's self-determination and the risks he took in visiting the occupied territories. De Marco also visited North and South Korea and saw to their admission to the UN in 1991. He pursued various initiatives between the UN, Europe and the Mediterranean and proposed a new role, which is still under discussion, for the UN's trusteeship council, with some of its aims being to aid and give purpose to communities and refugees from failed or wartorn states in extraterritorial zones; and to protect the resources of the sea and the seabed, the climate and the rights of future generations.

De Marco taught law at the University of Malta from the 60s onwards, published several legal textbooks and in the 80s was appointed professor of criminal law at the university. De Marco was also a spiritual, practising Catholic, who was much devoted to Our Lady of Mount Carmel. He forwarded Malta's request to join the EU on her feast day, 16 July. When I last spoke to him some six months ago, he remained upbeat and determined, despite health problems; nonetheless, he also expressed his gratitude to God "for every hour he lent".

He is survived by his wife, Violet, and his three children, Gianella, Fiorella and Mario.

• Guido de Marco, lawyer and politician, born 22 July 1931; died 12 August 2010